The present invention is directed to a method for improving the protection against corrosion of surfaces that are coated with aluminum or an alloy consisting predominantly of aluminum.
Magnets, in particular magnets that contain rare earth, for example NdFeB magnets, will exhibit a strong tendency to corrode due to their affinity to oxygen. During the use of parts of this type of magnet in atmospheres that promote corrosion, such as, for example, in an atmosphere with a high humidity or an atmosphere containing salt, it is thus necessary to protect the magnetic material by means of a coating that inhibits corrosion. For this purpose, besides lacquering processes, coatings with nickel, tin or aluminum are standard.
An article by T. Minowa entitled "Coating Technology on Nd Magnets" Gorham International Conference, Monterey, Calif., Feb. 26-28, 1989, discusses coatings to protect Nd magnets. A coating with aluminum can be provided by what is known as an ion plating method, which is also called IVD, wherein the IVD stands for "ion vapor deposition. Here, the aluminum is vaporized and is deposited on the surface of a negatively-charged part with a voltage of 1 kV to 5 kV. Besides this, the galvanic depositing of aluminum from anhydrous electrolyte solutions is possible. Depositing can also take place onto any other known type of aluminum layer on a magnet of this type, and also onto other parts.
It is also theoretically possible to use aluminum alloys rather than pure aluminum for coating, which alloys should then, however, consist predominantly of aluminum. Magnets with this sort of coating have a particular advantage against corrosion in atmospheres containing salt. In environments of this sort, aluminum is more resistant against corrosion than parts coated with, for example, nickel. For example, given a spraying with a solution of 5% NaCl, an endurance of less than 100 hours will result in a nickel-coated part, while the aluminum-coated part resists corrosion for more than 1000 hours.
The behavior is otherwise when the environment is not air containing salt, but rather is a particularly humid environment at a relatively warm temperature. In a corrosion test at an ambient temperature of 85.degree. C. and a relative air humidity of 85%, the typical results for nickel-coated magnets is an endurance of 1000 hours, while the endurance for an aluminum-coated magnet is only 500 hours. Similar behavior is found in what is called a HAST test, wherein HAST stands for "highly accelerated steam test". Here, an atmosphere containing steam is used with a temperature of 130.degree. C. at a pressure of 2.5 bar. In an atmosphere of this sort, a nickel-coated part has an endurance of 50 to 100 hours, while the endurance for an aluminum-coated part is less than 50 hours.
For many applications, in particular in the automotive field, a coating is, however, required that exhibits both endurance in humid environments and resistance to air containing salt. Requirements of this sort with regard to resistance to corrosion were previously met by manufacturing the parts with a double-coating. Here, for example a layer of greater than 20 .mu.m of nickel and a layer of greater than 20 .mu.m tin were deposited on the parts to be protected, for example, a NdFeB magnet.
However, a double-coating of this sort has the disadvantage that it requires a significantly higher economic outlay. Due to the necessarily greater thickness of the non-magnetic layers, the portion of the magnetically effective material relative to the total volume is lower, so that allowances have to be made with regard to magnetic characteristics in the magnetic system and allowances also have to be made in the tolerance that can be realized for the magnets.